1. Field of the Invention
This invention is in the field of telescope eyepiece design.
2. Description of Prior Art
The eyepiece of an optical telescope comprises a series of lenses to provide light refraction for magnification and focus. Refractive lenses produce several distortions and aberrations, including color separation, or chromatic aberration. The eye relief and the field of view of an eyepiece are limited by the correlation between the refractive power of the lenses and the magnitude of distortions.
Color distortion is caused by a difference in focal lengths for different colors of light passing through a lens. Higher frequencies of light (blues) are refracted more than lower frequencies (red). Thus, parallel rays of white light passing through a positive (convex) lens will separate into colors, with blues focussed shorter than reds.
Color distortion is counteracted in conventional eyepieces by using two or more lenses of different optical characteristics cemented together. For example, a convex-convex lens of crown glass can be cemented to a piano-concave lens of flint glass. Flint glass has a higher refractive index than crown glass, and much higher dispersion, or color separation. The two lens elements are matched such that the concave lens only partly counters the refraction of the convex lens, but fully counters its dispersion. The result is a net positive combined lens with no chromatism, called an achromatic doublet. Sometimes more than two lenses are cemented together to optimize the design for a particular use.
Numerous telescope eyepiece designs are available, including Huygens, Ramsden, Tolles, Hastings, Steinheil, Kellner, Orthoscopic, Plossl, Erfle, Aspheric, Bertele, Tele Vue Wide-Angle, Nagler types 1 and 2, and RKE. The simpler ones have a small field of view and noticeable aberrations, possibly including chromatism. The best-performing designs use achromatic doublets to correct many of these faults, but are expensive and often have faults anyway.
A characteristic of some eyepieces is a kidney-bean-shaped black cloud that appears in the image unless the eye is located precisely at the eyepiece. For example, the Nagler type 1 eyepiece has an 82-degree field of view, but exhibits "kidney bean" aberration, and it uses 7 lens elements with three doublets. The Nagler Type 2 eyepiece reduces this problem, but requires three doublets and a total of 8 lens elements. The Nagler eyepieces also have pincushion distortion.
Curvature of field is a substantial disadvantage for older viewers, since eye accommodation lessens with age due to hardening of the lens in the eye. Curvature of field in an eyepiece thus causes the edge of the field to be out of focus for an older person, limiting the useful field without an eyepiece focus adjustment. A young person's eye can easily change diopter enough to quickly adjust focus as the view goes from center to edge. Thus, a young viewer is hardly aware of the eyepiece curvature of field. Astigmatism toward the edge of the field reduces resolution. Many people, especially beginners, are content to live with off-axis aberrations like curvature and astigmatism, especially if the eyepiece is a bargain in price. For a sharper image, one can center the object in the eyepiece.
Inadequate eye relief (distance of the eye from the lens) is a problem on some eyepieces, allowing the eyelashes to brush the eye lens, coating it with body oils, which quickly reduces resolution. A Kellner has very good color correction. If the elements are multicoated, it is probably as good optically as an Orthoscopic, but the eye relief is small. One sweep of the eyelashes and the crispness of view is lost. This is why many astronomers prefer Orthoscopics, which allow viewing all night without having to clean the eyepiece. However, the apparent field of view is only about 40 degrees in both the Kellner and Orthoscopics, so they are best suited for planetary viewing. The RKE (Edmund) is a design based on the Kellner with better eye relief, but it still has a small field. A small field can be an advantage for planetary viewing because extraneous items are automatically eliminated. However, for general viewing, once a person looks through a good wide-field eyepiece, he will probably be dissatisfied with anything under about a 60-degree field. Narrower fields give the impression of looking through a tunnel. It is better to have a good wide-field eyepiece with an adjustable aperture which can be reduced to concentrate on one object for a period of time.
The Kellner 3-element eyepiece (with one doublet) has good color correction and fair edge sharpness, but has appreciable astigmatism and curvature of field with only a 40 to 45 degree field width. The Orthoscopic 4-element eyepiece (with one triplet) has high image contrast and very good color correction, but has appreciable field curvature and astigmatism at the edge of a 40 to 45 degree field. The Plossl 4 to 7 element eyepiece has good image sharpness across 36 to 52 degrees, and perfect achromatism, but has noticeable astigmatism starting 15 degrees from the center of the field, and it uses doublets. The Pentax XL, Meade Super WideAngle, and TeleVue Panoptic 5 to 8 element widefields have 60 to 70 degree fields and excellent image sharpness and contrast. The Naglers and Meade Ultra Wide Angle 7 or 8 element have fields of 80 degrees or more. However these wide and ultrawide field eyepieces use doublets, usually have noticeable distortion, some kidney bean aberration, and their resolution is noticeably reduced below f/4.5 or f/4. They are also very heavy, and may not be capable of incorporating the atmospheric dispersion correction of U.S. Pat. No. 5,696,635, but may require a different more costly approach.
Wide-field eyepieces are becoming more popular. If it were not for the cost, these are what most viewers would probably choose, not only for the wide field, but because the image looks sharp and more natural over the whole field. Inexpensive eyepieces often have fields effectively narrowed by aberrations toward the edges, such as field curvature, distortion, astigmatism, and excessive color. Such eyepieces may only provide 25-30 degrees of useable field width, although additional unusable field width is sometimes provided for exaggerated advertising claims. However, present widefields are excessively large, heavy, and costly.
Achromatic doublets are more expensive than simple lenses, due to the additional lens elements, the design work and precision required to match a common surface, and the assembly steps of gluing them together. They are also heavier than simple lenses. It would be an advantage to eliminate them if chromatism could be avoided while providing high refractive control, a wide field of view, and avoiding other distortions and aberrations.